The present invention relates to a rear-projection screen and, more particularly, to a screen used as the display of a rear-projection TV using a liquid crystal panel as a video source.
FIG. 7 is a sectional plan view showing a conventional rear-projection screen generally used for a CRT rear-projection TV. This screen comprises, from the projection side (rear side), a Fresnel lens sheet 110, double-side lenticular sheet 120, and protective plate 130.
The Fresnel lens sheet 110 has a transparent resin substrate 111, and Fresnel lens 112 on its front side. The protective plate 130 has a transparent resin substrate 131 and, normally, a surface-treated layer 138 formed on its front side. The protective plate 130 prevents dust from sticking to the three-dimensional surface of the double-side lenticular sheet 120 on the front side and protects a light-shielding layer 125. The surface-treated layer 138 of the protective plate 130 is formed as an abrasion-resistant hard coat or an antistatic treatment.
The lenticular sheet 120 has a resin substrate 121 and lenticular portions 122 and 123 on both surfaces of the substrate. The lenticular portion 122 on the rear side is constructed by horizontally juxtaposing a plurality of cylindrical lenses extending in the vertical direction. The lenticular portion 122 is provided to refract/diffuse video light in the horizontal direction. The lenticular portion 123 on the observation side (front side) is also constructed by horizontally juxtaposing a plurality of cylindrical lenses extending in the vertical direction. The lenticular portions 122 and 123 have the function of correcting a color shift (to be described later).
A light diffusing material 124 is mixed into the resin substrate 121 of the lenticular sheet 120 to refract/diffuse video light in the vertical direction. The resin substrate 121 has on its front side protruding portions alternating with the cylindrical lenses of the lenticular portion 123. Each protruding portion has a flat top on which a light-shielding layer 125 is formed by printing black ink or the like. The light-shielding layer 125 is provided to improve external light contrast.
Recently, CRT rear-projection TVs are being replaced with liquid crystal rear-projection TVs with characteristic features such as small power consumption, high-image-quality, and lightweight/low-profile structure. In a CRT rear-projection TV, an image is projected from three tubes of R, G, and B, and the positional difference between the three tubes generates a color shift. To correct the color shift, the rear-projection screen must have a double-side lenticular sheet. However, in a liquid crystal rear-projection TV, video light from the liquid crystal is projected through a monocular lens, and no color shift due to the position difference occurs. Hence, the rear-projection screen can use a single-side lenticular sheet.
FIG. 8 is a sectional plan view showing a conventional rear-projection screen used for a liquid crystal rear-projection TV. This screen comprises, from the rear side, a Fresnel lens sheet 140 and horizontal lenticular sheet 150.
The Fresnel lens sheet 140 has a resin substrate 141, and Fresnel lens 142 on its front side. The lenticular sheet 150 has a resin substrate 151, lenticular portion 152 on its rear side, and surface-treated layer 158 on its front side. The lenticular portion 152 is constructed by horizontally juxtaposing a plurality of cylindrical lenses in the vertical direction. The lenticular portion 152 is disposed to refract/diffuse video light in the horizontal directions
This screen has no light-shielding layer as shown in FIG. 7, so low contrast due to external light poses a problem. To refract/diffuse video light in the vertical direction and increase the contrast, light diffusing material 154 and dark colorant 154 are mixed into the resin substrate 151 of the lenticular sheet 150. The effect provided is not sufficient, and therefore, a light diffusing material 144 is mixed into the resin substrate 141 of the Fresnel lens sheet 140.
A considerably large amount of light diffusing materials 144 and 154 and dark colorant 154 need be added to obtain a sufficient addition effect. As a result, the screen luminance becomes low in proportion to the content of the added materials. If the content is small, the video light refraction/diffusion function in the vertical direction is insufficient, resulting in a narrow vertical viewing angle of the screen.
FIG. 9 is a sectional plan view showing another example of the conventional rear-projection screen used for a liquid crystal rear-projection TV. This screen comprises, from the rear side, a Fresnel lens sheet 160 and horizontal lenticular sheet 170.
The Fresnel lens sheet 160 has a resin substrate 161, and Fresnel lens 162 on its front side. A light diffusing material 164 is mixed into the resin substrate 161 of the Fresnel lens sheet 160.
The horizontal lenticular sheet 170 has a resin substrate 171, and lenticular portion 172 on its rear side. The lenticular portion 172 on the rear side is constructed by horizontally juxtaposing a plurality of cylindrical lenses extending in the vertical direction. A UV curing resin layer 176 and light-shielding layer 175 are formed on the front side of the resin substrate 171. The light-shielding layer 175 consists of black stripes formed in regions except the light focusing portions of the cylindrical lenses of the lenticular portion 172.
The horizontal lenticular sheet 170 also has a protective resin substrate 181 bonded to the front side of the light-shielding layer 175 via an adhesive layer 177. A light diffusing material 184 and dark colorant 184 are mixed into the resin substrate 181. A surface-treated layer 188 is formed as an anti-reflection treatment, antistatic treatment, and hard coat treatment on the front side of the resin substrate 181.
According to the screen shown in FIG. 9, the contrast is improved, and the content of the above-described colorant or light diffusing material can be reduced because of the effect of the black light-shielding layer 175. In this case, however, a phenomenon called "hot bar" inherent to the liquid crystal rear-projection TV is observed. In the "hot bar" phenomenon, light from the projector light source is observed locally bright, on the screen surface, as a long bar which extends in the horizontal direction perpendicular to the length of the cylindrical lenses of the lenticular portion 172. To solve this phenomenon, light diffusing materials and colorants 164 and 184 are mixed into the resin substrate 161 of the Fresnel lens sheet 160 and the protective resin substrate 181, respectively, to subject them to a light diffusion treatment. For this screen, however, the screen luminance lowers, and the manufacturing cost increases because of preparation of the resin substrates 161 and 181.